They would get better reception if they raised taxes across the board, and then offered a tax break for families with immunized children. Exactly the same result, but it's phrased as a reward rather than a punishment.
Same scheme would work in other contexts too:
If Obama's health care plan gave people a tax break to people with health insurance, instead of fining people without it, then there would be no danger of a constitutional challenge.
Airlines are always aiming for the smallest list price, but given how people feel about them they could really take an alternative tack: include everything in the cost, but give discounts to people without checked luggage, people who sit in the cramped seats, people who are willing to board the plane last, etc. All of a sudden, you're the airline that is *giving* money away, instead of nickel-and-diming everybody, so that even if your prices are a little higher you will have built up goodwill.